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Science: Lasers illuminate chemistry of vision

23 November 1991

By JIM BAGGOTT

For more than 20 years, chemists have known that the first step in vision
involves a fast chemical reaction initiated by light. Now researchers in
the US have used very short pulses of light to determine just how fast this
reaction is. They have found that it occurs in 200 million billionths of
a second, making it one of the fastest reactions ever observed experimentally
(Science, vol 254, p 412).

Researchers from the University of California’s chemistry department
and its Lawrence Berkeley Laboratory studied rhodopsin, extracted from bulls’
eyes. The molecule is made up of a protein and a long-chain molecule called
retinalis, and is found in some cells of the retina.

When retinalis absorbs a photon of light, it twists around a carbon-carbon
double bond in the chain. This is the first step in a convoluted series
of biochemical changes which turn light into an electrical signal which
is passed to the brain along the length of the optic nerve.

The American chemists were able to study the reaction because of recent
developments in ultrafast lasers. The reaction was so quick that it challenges
the traditional picture of how photochemical reactions occur, they say.